This invention relates to the manufacture of semiconductor devices and, more particularly, to heating apparatus for practicing temperature gradient zone melting during the manufacture of semiconductor devices.
In the manufacture of semiconductor devices, it is usually necessary to alter or tailor the conductivity type of a body of semiconductor material. This alteration is provided by distributing atoms of conductivity modifying dopant in a selected region or selected regions of the body. Several techniques for achieving that distribution are widely practiced today. For example, doping is provided by alloying, diffusion, epitaxial growth, etc. The choice among the various methods is made on the basis of such considerations as cost and the character of the junction desired. Alloying, for example, provides very sharp but shallow junctions. Diffusion can provide a deeper junction, but it is less sharp.
Temperature gradient zone melting is a method of doping semiconductor material that can provide very abrupt junctions coupled with high dopant concentrations. Specifically, temperature gradient zone melting provides doped regions containing the solid solubility limit of the dopant. Another virtue of temperature gradient zone melting is that doped regions of unusual configurations can be provided.
Early descriptions of temperature gradient zone melting and some of its applications will be found in U.S. Pat. No. 2,813,048 issued to W. G. Pfann, and in his book Zone Melting, copyright 1958 by John Wiley & Sons, Inc.
While temperature gradient zone melting was known as a method of doping semiconductors very early in the life of the semiconductor industry, certain heretofore unsolved problems have prevented its adoption as a standard tool of the semiconductor device design engineer.
Certainly one of the most difficult to surmount problems has been properly heating the body of semiconductor material. The body of semiconductor material must be heated to an elevated temperature and must have a temperature gradient impressed thereacross. For reasons that will become clear below, the position, direction and uniformity of that gradient is crucial to the successful practice of temperature gradient zone melting.
Several methods of heating have been tried. For example: E-beam, vacuum melting, and placing a workpiece directly on a heated body have all been tested. However, none of these methods has proven completely successful, and each has had its own problems. For example, E-beam is an effective way of heating small areas but to adequately heat an entire semiconductor wafer utilizing E-beam, scanning must be employed and that is difficult to do evenly. A disadvantage associated with vacuum melting is that dopants exhibiting a high vapor pressure at the temperatures employed cannot be used. When heating by contact with a heated body, it has been found difficult to assure uniform contact over the entire wafer area. Thus, hot spots and undesirable transverse temperature gradients will be present.
In summary, no completely satisfactory heating technique for the practice of temperature gradient zone melting on a commercial scale has heretofore been developed.
It is, therefore, an object of this invention to provide an apparatus for heating bodies of semiconductor material during the practice of the temperature gradient zone melting technique which rendered the process commercially viable.